Method and apparatus for measuring the condition of degradable components

ABSTRACT

An apparatus and method for the in-situ measurement of the condition of degradable components such as electrical cable insulation, valve internals, and gaskets. A stream of energetic subatomic particles is directed to the component under test, thereby inducing the emission of secondary gamma radiation which is detected by one or more radiation detectors positioned in proximity to the component. The secondary radiation emission spectrum is recorded and analyzed to identify features and/or changes resulting from the application of one or more stressors to the component. In the specific case of aging, the radiation spectra taken from the same component at different intervals during its lifetime are compared to identify changes in the component which then may be correlated with artificially (or naturally) aged specimens to estimate the relative level of aging of the component.

This application is a continuation of U.S. patent application Ser. No.09/074,207 filed May 7, 1998 entitled “Method and Apparatus forMeasuring the Condition of Degradable Components”, now U.S. Pat. No.6,144,032.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of material aging researchand management, specifically to the in-situ monitoring and estimation ofthe condition of various degradable components used in a wide variety ofapplications (including, inter alia, electrical cable, process systemvalves, aircraft, spacecraft, and automobiles) via neutron activationtechniques.

2. Description of Related Technology

The aging of degradable components (particularly those constructed inwhole or in part of organic compounds such as polymers) is of greatimportance to modern society. Such degradable components comprise asignificant fraction of what may be termed as “critical” components inuse in many industrial, aerospace, and automotive applications, bothcommercial and military. Included in this category are components suchas electrical cable insulation, valve internals, bushings, seals, andgaskets. Degradation and ultimate failure of these so-called criticalcomponents is of paramount importance in that such failures may resultin the unanticipated maintenance costs, loss of operational capabilityand availability, and even loss of human life.

Several different approaches to managing the aging of such componentsexist. One approach involves 1) subjecting laboratory or in-situspecimens of a given component to a progressive regimen of agingstressors such as heat, radiation, electrical potential, chemicals,and/or oxygen present in the anticipated operating environment (knowngenerally as “artificial aging”); 2) identifying a critical parameter ofthe component's function in the desired application (such as dielectricstrength for an insulator); 3) determining a maximum or minimumacceptable value for the chosen parameter; 4) correlating the maximum orminimum acceptable value to a given installed lifetime (for example, viaaging models such as the Arrhenius equation); and 5) removing thecomponent from service when the installed lifetime is reached. Note,however, that this approach has the distinct disadvantage of notdirectly monitoring the condition of a given component, therebyintroducing potentially significant variations in component conditionacross various applications. Specifically, some applications may haveaged more or less than expected (due to a variety of factors such asradiant heat or radiation shielding, variations in oxygen/inert gasconcentration, aging prior to installation, inaccuracies in the agingmodel used, etc.), and hence are being replaced either prematurely ortoo late. More effective condition monitoring programs will utilize asimilar approach as that outlined above, yet instead of rotely replacinga component at a given point in life, will monitor the degradation ofthe component as a function of time to determine it's rate of aging ascompared to the artificially (or naturally) aged specimen. The primarydrawbacks of these latter condition monitoring programs include thecosts of monitoring, component inaccessibility, and component/devicedowntime. For example, the condition monitoring of a fluoropolymer valveseat requires either remote inspection or disassembly of the valve,thereby removing the valve from service for a period of time. In suchcases, simple periodic replacement of the component during otherscheduled maintenance may be more cost effective. In some instances(such as electrical cable, described further below), no periodicmaintenance or replacement is ever scheduled; hence condition monitoringof some sort is almost a necessity. The enormity of cost associated withreplacement of cable in, for example, a commercial nuclear powerfacility, underscores the need for effective aging assessment andmonitoring techniques.

Electrical Cable

As previously indicated, the aging and unanticipated failure of power,control, instrumentation, and data transmission cable may havesignificant adverse effects on plant operation and maintenance (O&M)costs and downtime. Electrical and optical cables have traditionallybeen considered long-lived components which merit little in the way ofpreventive maintenance or condition monitoring due to their generallyhigh level of reliability and simplicity of construction. Like all othercomponents, however, such cables age as the result of operational andenvironmental stressors. Aging effects may be spatially generalized(i.e., affecting most or all portions of a given cable equally, such asfor a cable located completely within a single room of uniformtemperature), or localized (i.e., affecting only very limited portionsof a cable, such as in the case of a cable routed near a highlylocalized heat source). The severity of these aging effects depends onseveral factors including the severity of the stressor, the materials ofconstruction and design of the cable, and the ambient environmentsurrounding the cable. Detailed discussions of electrical cable agingmay be found in a number of publications including SAND96-0344 “AgingManagement Guideline for Commercial Nuclear Power Plants—ElectricalCable and Terminations” prepared by Sandia National Laboratories/U.S.Department of Energy, September 1996. Discussions regarding opticalcable aging may be found, inter alia, in Electric Power ResearchInstitute (EPRI) publications and telecommunications industryliterature. The following description will be limited to electricalcable, although it can be appreciated that the principles of aging andanalysis described herein may also be largely applicable to opticalcabling as well as many other types of polymeric components.

Electrical cables come in a wide variety of voltage ranges andconfigurations, depending on their anticipated uses. Existing prior artlow- and medium-voltage power and control cables such as that shown inFIGS. 1a-1 d are typically constructed using a polymer or rubberdielectric insulation 200 which is applied over a multi-strand copper oraluminum conductor 202. The insulation is often overlaid with aprotective polymer jacket 204. In multi-conductor cables (such as thoseused in three-phase alternating current systems, as shown in FIGS. 1aand 1 b), a plurality of these individually insulated conductors areencased within a protective outer jacket 206 along with other componentssuch as filler 208 and drain wires (not shown). These other componentsfulfill a variety of functions including imparting mechanical stabilityand rigidity to the cable, shielding against electromagneticinterference, and allowing for the dissipation of accumulatedelectrostatic charge. This general arrangement is used for itsrelatively low cost, ease of handling and installation, comparativelysmall physical dimensions, and protection against environmentalstressors.

Current methods of evaluating electrical cable component aging generallymay be categorized as electrical, physical, and microphysical.Electrical techniques involve the measurement of one or more electricalparameters relating to the operation of the cable, such as the breakdownvoltage, power factor, capacitance, or electrical resistance of thedielectric. These methods have to the present been considered largelyineffective or impractical, in that they either do not show a goodcorrelation between the parameter being measured and the aging of thedielectric, or are difficult to implement under normal operations.Furthermore, such techniques are often deleterious to the longevity ofthe insulation, and have difficulty determining localized aging within agiven conductor.

Physical techniques including the measurement of compressive modulus,torsional modulus, or rigidity under bending often show a bettercorrelation between the aging of the cable and the measured parameter(especially for low-voltage cable), and are more practical to applyduring operational conditions. However, they generally suffer from alack of access to the most critical elements of the cable, theindividual electrical conductors and their insulation. For example, themeasurement of compressive modulus by way of instruments such as theIndenter Polymer Aging Monitor are effective primarily with respect tothe outer, accessible surface of the cable such as its outer jacket.Although correlations of the aging of the outerjacket to that of theunderlying conductors have been attempted, these correlations aregenerally quite imprecise and are subject to a large degree ofvariability based on the specific configuration of the cable beingtested (i.e., its materials of construction, insulation/jacketthickness, etc.), the presence of ohmically induced heating, shieldingof the conductors against stressors by the outer jacket, and differencesin the oxygen concentration at the conductor insulation versus that atthe outer jacket. See EPRI TR-104075, “Evaluation of Cable Polymer AgingThrough Indenter Testing of In-Plant and Laboratory Aged Specimens,”prepared by the Electric Power Research Institute, January, 1996 for adiscussion of the correlation between outer jacket and conductorphysical measurements.

Other physical techniques such as the measurement of the tensilestrength or elongation-at-break of the insulation material areinherently destructive and require a specimen of the aged cable fortesting.

Another potential drawback to many of the physical techniques describedabove is disturbance of the bulk cable run during testing. In someapplications, the dielectric of the cable being evaluated may be highlyaged and embrittled, yet still completely functional. However,substantial movement of the cable (such as picking the cable up andclamping on a test device) may produce localized elongation stressesbeyond those corresponding to the elongation-at-break for the insulationand/or jacket material, thereby inducing unwanted cracking of theinsulation and/orjacketing and potential electrical failure.

Microphysical techniques such as the measurement of insulation oxidationinduction time (OIT), density, gel or plasticizer content, infraredabsorption spectroscopy UV spectroscopy, and NMR are generally quiteaccurate, yet require samples of the cable insulation and/or jacket foranalysis. For jacketed conductors, such samples are generally onlyavailable at the ends of the cable where the conductors are terminatedto a source or load, and not anywhere between. Furthermore, as with thephysical techniques described above, the results of any such testing arenecessarily applicable only to the localized area of the cable fromwhich the specimen was taken, which may or may not be representative ofthe rest of the cable. Hence, one can either take a small sample ofmaterial from the outer jacket of the cable and attempt to extrapolatethe results of the aging analysis to the underlying conductors, oralternatively take a sample at the ends of the conductor itself near itsterminations and extrapolate these results to the rest of the unexposedconductor. Under either alternative, a substantial degree of uncertaintyand imprecision exists. Plant operators are also generally reticent toallowing the removal of even small samples of material from theircables, especially in applications where plant safety and continuity ofelectrical power are critical.

Another common problem in applying either physical or microphysicaltechniques to a localized portion of cable is the existence of conduit.In the typical power or industrial plant, many miles of cable may beencased within metallic or plastic conduit, thereby rendering it all butinaccessible. While it is true that such conduit also affords the cableadditional protection from most stressors (such as heat and radiation),it also may preclude any effective estimation of aging using existingtechniques. For example, the aging of a portion of nuclear plantsafety-related cable contained in a conduit running directly over alarge radiant heat source may be for all intents and purposesimmeasurable during it's installed lifetime. While the remainder of thecable not in direct proximity to the heat source may be largelyunaffected, the insulation of the cable in the region directly adjacentto the heat source may undergo dramatically accelerated aging andultimately failure well in advance of the rest of the cable.

Fast Neutron Activation

The technique of fast neutron activation (FNA) is well known in thenuclear arts. Generally speaking, this technique employs a stream ofenergetic (fast) neutrons to induce secondary gamma ray emission from atarget object via inelastic scattering with nuclei in the target. Thegamma ray spectrum associated with a given element is unique andidentifiable given sufficient energy resolution. Heretofore, FNA systemshave been used exclusively in the detection and identification analysisof organic materials in obstructed locations (such as in contrabanddetection or bore hole exploration; see for example U.S. Pat. No.5,098,640, “Apparatus and Method for Detecting Contraband using FastNeutron Activation”). Such techniques, however, have not been applied tothe in-situ analysis of changes in the atomic structure of a materialresulting from the application of stressors (such as heat, nuclearradiation, oxygen/ozone, etc.). Furthermore, existing neutron scanningand detection systems necessarily utilize very high neutron fluxes(>1E10 n/s-4π) in order to minimize analysis time. Such systems caninduce significant damage to both inorganic (such as metals) and organicmaterials. While neutron radiation primarily results in atomicdisplacement effects (which are highly detrimental to inorganics), italso induces a substantial degree of ionization within organicmaterials.

Based on the foregoing, it would be most desirable to provide anapparatus and method which allows an operator to more accurately assessthe aging an in-situ degradable component in a substantiallynon-destructive manner and without requiring direct access to thecomponent. Such apparatus and method could, for example, be used toestimate the aging of an electrical cable within a metallic conduit, orsimilarly to estimate the aging of a valve internal component whilestill installed within its host valve.

SUMMARY OF THE INVENTION

The present invention satisfies the aforementioned needs by providing animproved apparatus and method for the in-situ estimation of polymericcomponent degradation and aging.

In a first aspect of the invention, a collimated stream of energetic(“fast”) neutrons generated by a neutron source is used to bombard thesubject in-situ degradable component in order to induce inelasticscattering with various constituent atoms in the materials ofconstruction. Such inelastic scattering results in the production ofgamma rays of varying energy (the energy being dependent in part on theidentity of the scattering atom). One or more gamma ray detectors areplaced in proximity to the irradiated component to measure the resultinggamma ray spectra during bombardment. Since the relative concentrationsof various constituent atoms within certain component material(s) changeas a function of aging, the gamma emission spectra from the componentwill also change with aging. Scattering resulting from neutroninteraction with metal atoms or other essentially aging-independentmaterials (such as those in the conductor, shield, or conduit of anelectrical cable, for example) will remain effectively constant, andtherefore is easily differentiable from scattering associated withage-variant atomic concentrations such as plasticizers or fireretardants present in the polymers.

In a second aspect of the invention, an improved method for estimatingthe aging of a degradable component using the previously describedapparatus is disclosed. Gamma emission spectra of an in-situ testcomponent are taken at various times during its installed lifetime, andcompared to each other as well as other spectra obtained fromlaboratory-aged specimens of similar components. In one embodiment, theanalog gamma emission spectrum is converted to a digital representationusing an analog-to-digital converter (ADC), electronically filtered, andthen subtracted from prior spectra to generate “difference” spectra forthe component under test. Such difference spectra are compared to thosederived from known aged specimens, and may further be analyzed andcompiled to generate a statistical model of aging within a given type ofcomponent. In this fashion, the relative level of aging of the in-situcomponent can be reliably estimated at any given point during itslifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 d are perspective and cross-sectional views of two typicalprior art electrical cables (3φ ac and 2-conductor dc, respectively),showing the cable conductors, insulation, shielding, and jacket.

FIG. 2 is perspective view of a first embodiment of the conditionmonitoring apparatus of the present invention.

FIG. 2a is a perspective view of a first embodiment of the collimator ofthe condition monitoring apparatus of FIG. 2.

FIG. 3 is a top cross-sectional view of a second embodiment of thecondition monitoring apparatus of the present invention.

FIG. 3a is a side cross-sectional view of a second embodiment of thecondition monitoring apparatus of the present invention, taken alongline 3 a—3 a of FIG. 3.

FIG. 4 is a schematic block diagram of a first embodiment of theanalyzer of the present invention, showing the detector channels andsignal processing equipment.

FIG. 5 is a functional representation of the X and Y array structureused in conjunction with the analyzer of FIG. 4.

FIGS. 6a and 6 b illustrate two typical gamma spectra obtained from anin-situ specimen at successive aging intervals.

FIG. 7 is a functional flow chart illustrating one embodiment of theaging estimation method of the present invention.

FIG. 8 illustrates a typical difference spectrum generated bydifferencing the gamma emission spectra of FIGS. 6a and 6 b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to the drawings wherein like numerals refer tolike parts throughout.

While the following description is made primarily with reference toelectrical cable, it can be appreciated that many of the aspects of thepresent invention may generally be adapted to use on other types ofcomponents and devices including without limitation, optical cable,valve internals, and automotive or aircraft engine components such asgaskets or seals. Furthermore, analyzed components need not necessarilybe polymeric in composition, but rather may be comprised of any materialwhose gamma emission spectrum varies measurably as a function of theaging of (or other stressors applied to) the material. Finally, whilethe use of neutrons and gamma rays are described in detail, the use ofother forms of radiation (both incident and secondary) for componentdegradation evaluation is contemplated by the invention.

FIG. 2 shows a first embodiment of the improved condition monitoringapparatus of the present invention. A neutron source 12 is positionedgenerally in relative proximity to the subject 11 being evaluated, inthe present case an electrical cable installed within a conduit (seeFIG. 1). The source 12 may be of any readily available type whichproduces sufficiently energetic (i.e., typically between 5 and 15 MeV)neutrons in quantities necessary to generate the desired net neutronflux 13 after collimation (described further below). A Kaman Nuclearparticle generator utilizing a deuteron/tritium beam having 14 MeVneutrons is used as the source 12 in this embodiment, although it can beappreciated that other types of sources (such as those employing adeuterium/deuterium, deuterium/beryllium, or hydrogen/lithium beam) maybe used with equal success. Furthermore, although the neutron source 12of the present embodiment is continuous in nature, pulsed sources mayalso be employed, depending on the needs of the intended application.Pulsed fast neutron sources and systems are well known and understood inthe art; see for example, U.S. Pat. No. 5,076,993, “Contraband DetectionSystem Using Direct Imaging Pulsed Fast Neutrons.”

In the present embodiment, fast neutrons having energies on the order of14 MeV are utilized, although neutron energies outside this range mayalso be used depending on the specific application. For example, thermalneutrons (<<1 MeV) may be particularly useful for the detection ofcertain atoms such as nitrogen. The present invention contemplates theuse of either fast or thermal neutrons, or both. Note that within acertain range of neutron energies, 1) the production of gamma rays atcertain energies is significantly enhanced in certain materials, and 2)the energy of gamma rays resulting from neutron scattering areessentially constant. A practical consequence of varying neutron energyis the change in probability of creation of a given gamma event. Neutronflux is also a determinant of gamma ray production; higher incidentneutron flux will, holding all else constant, generally produce a highergamma flux.

A desirable characteristic of the neutron source 12 is comparativelysmall size and relative portability to permit in-situ testing ofcomponents. However, since the scattering cross-section of energeticneutrons in air is quite low, the source may be placed somewhat remotelyto the test subject if desired without appreciable reduction inefficiency. The source of the present embodiment is fitted with acollimator 14 on its outlet which spatially collimates the neutron beam13 from the source 12 prior to reaching the test subject. The collimator14 may constructed of any material which effectively attenuatesenergetic neutrons, such as polyethylene (formulated with or withoutadditives such as boron) or lead. In the present embodiment,polyethylene is chosen due to its comparatively light weight, ease ofmanufacturing, low cost, and high neutron absorption/scatteringcross-section.

As shown in FIG. 2, the collimator 14 reduces the exposure ofsurrounding portions of the test subject to the neutron beam 13, andspatially refines the beam such that more precise testing of specificcomponents can be performed. For example, as shown in FIG. 2a, the beammay be collimated disproportionately in the X and Y planes such that anexposure slit 15 is formed. In this fashion, a “slice” of a test subject11 (electrical cable in this case) may be tested. Similar to techniquesemployed in prior art axial tomography, data from several of such slicesmay be electronically fused by the analyzer 60 to provide a spatialrepresentation of the aging of the cable. Alternatively, the neutronbeam 13 may be collimated to a tightly focused beam of essentiallycircular cross-section to allow examination of a very precise areawithin a structure (such as the stem seal of a valve).

Referring again to the embodiment of FIG. 2, one or more gamma raydetectors 16 are mounted on separate articulated arms 17 attached to asupporting frame element 18, the latter being which is moveable inrelation to the source 12 and its pedestal 21. In this fashion, a broadrange of detector positions relative to the source 12 can be achievedsuch that optimal test efficiency and adaptability can be supported. Forexample, the detector arms 17 may be positioned relative to the testsubject 11 such that the maximum gamma event rate is achieved for agiven neutron flux and energy. The frame element 18, detector arms 17,and articulated joint(s) 19 shown in FIG. 1 may be constructed in anynumber of well known and understood configurations, and fabricated usingany suitable material such as steel, aluminum, or polymer.

The gamma detectors 16 of the present embodiment are neutron shieldedhigh purity Germanium crystal detectors of the type well known in theart, the theory of operation of which is described in further detailbelow. Such detectors have the important advantage of high spectralresolution (typically less than 1%) as compared to other types of gammadetectors. It should be noted, however, that any type of gamma raydetector having sufficiently high spectral resolution may be used in thepresent invention.

In addition to the collimator 14, neutron shielding 22 and gamma rayshielding 24 is optionally utilized to shield the equipment operator,nearby personnel, and equipment from the relatively high neutron andgamma radiation fluxes generated by the apparatus 10. The primary objectof the neutron and gamma shields is to allow testing of components in atypical setting or environment (such as a nuclear power plant) which maybe populated during testing. In this fashion, elaborate precautionsassociated with high dose-rate/high energy radiation sources (such asthose utilized during radiography) can be largely obviated. It should benoted however that while highly desirable, neutron and gamma shielding22, 24 are not essential components of the present invention. Forexample, the equipment operator can be located remotely and areasadjacent to the test location evacuated as necessary to eliminate anypotential hazards to personnel resulting from neutron/gamma exposure.

A second embodiment of the condition monitoring apparatus of the presentinvention is shown in FIG. 3. The neutron shield 22 of this secondembodiment is a two-piece device having 1) a hollowed cylinder 23 withshield extension element 25 attached thereto, the shield element 25adapted to the test subject 11 shape (in the present case, a sectionedcylinder for use with an electrical cable conduit), and providingprotection against backscattered or deflected neutrons; and 2) abackstop element 26 having a similarly adapted shape. The backstopelement 26 is mounted to the shield element 25 via a hinge or similardevice thereby permitting the rapid positioning of the apparatus 10around the test subject 11. The material of construction neutronshielding 22 in the present embodiment is again chosen to bepolyethylene, although it can be appreciated that other types ofmaterials may be used. The shield element 25 and cylinder 23 alsocontain optional gamma shielding elements 27 to limit exposure of theneutron source 12, operator, and electronics to gamma exposure resultingfrom neutron irradiation. These gamma shield elements 27 are constructedof material similar to the main gamma shield 24 since the attenuation ofMeV-energy gammas by polyethylene is generally poor.

As shown in FIG. 3, two gamma detectors 16 are mounted within recesses32 within the neutron shield element 25 adjacent to the gamma shield 24.To minimize the size and weight of the gamma shield (which isappreciably more dense than the neutron shield 22), the gamma shield isplaced within the neutron shield elements 25, 26, and penetration 28 arecut into the gamma shield 24 to permit the maximum system count rateefficiency. The gamma shield 24 is comprised of a plurality of steel orlead interlocking components which effectively shield the majority ofthe solid angle (4π) around the test subject 11. The gamma shield 24also contains a neutron beam penetration 29 which allows passage of theneutron beam 13 from the source 12 to the test subject 11. Gamma“streaming” through the detectors and penetrations may be mitigatedthrough the use of a lead blanket, if desired, or the detectors 16 each“capped” with a form fit element 35 as shown in FIG. 3. It can also beappreciated that while two detectors are shown in the presentembodiment, any number of detectors may be utilized depending on thespecific application.

Note that for both the neutron and gamma shields 22, 24, lateralradiation streaming (i.e., out the sides of the shields longitudinallyalong the cable 11) is minimized in part by the cable and conduit (ifany). This is due largely to the construction of these components: thecable generally consists of a metallic conductor(s), with polymerinsulation and jacketing, while the conduit is often metallic inconstruction (typically either aluminum or steel). Hence, as the neutronand gamma shields 22, 24 are made to extend laterally from the neutronbeam impact point 37 on the cable 11, the amount of shielding providedby the cable and conduit is increased, since the effective gamma andneutron shielding thickness increases for all solid angles.

FIG. 3a shows a cross-section of the system taken perpendicular to thelongitudinal axis of the cable 11, illustrating the relative locationsof the above-described components.

Gamma Ray Spectral Analyzer

Referring now to FIG. 4, gamma rays detected by the gamma detectors 16described above are processed by the system analyzer 60 in order toproduce gamma energy spectra useful for the evaluation of componentcondition and aging. Generally, the analyzer consists of multipledetector channels having the following major components: 1) a pulseshaper 62; 2) an analog-to-digital converter (ADC) 64; 3) a pulse heightdiscriminator 66; 4) a first stage FIFO buffer 68 and associated buffermanager 70; 6) a logic gate 72; 7) a local memory 74; 8) a second stageFIFO buffer 76 and associated buffer manager 78 and 9) a personal orlaptop computer 80. The function and operation of each component isdescribed below. Note that while one specific embodiment of analyzer 60is described herein, any type and configuration of electronic signalanalyzer which performs the desired functions (i.e., gamma ray spectralprocessing) may be used without departing from the spirit of theinvention. For example, a conventional multi-channel analyzer (MCA) andnuclear scalers may be used with equal success.

As previously described, high purity crystalline scintillation detectors16 are used for the detection of gamma rays in the present invention. Ina scintillation detector, a gamma of a given energy excites crystal toproduce lower energy quanta (lower frequency electromagnetic radiation).These lower energy quanta are subsequently detected by a photomultiplier(PM) tube, the output of which is an analog signal representing thedetected gamma events. Specifically, the output of the PM tube is aseries of analog pulses, each pulse corresponding to a gamma detectionevent and having an amplitude essentially proportional to the energy ofthe detected gamma.

Separate detector channels 82 (as opposed to a common or multiplexedarrangement) are utilized in the embodiment of FIG. 4 to, inter alia,allow single detector processing, increase the system efficiency, andallow coincident or near-coincident gamma detection events occurringwithin different detectors 16 to be counted by the circuitry. Note thatcoincidence circuitry for the detector channels 82 is not required inthe present embodiment, since no neutron/alphaparticle or neutron/gammacorrelation is performed. However, utilization of such techniques (aswell as neutron time-of-flight) for spatial resolution within the testsubject 11 is contemplated by the present invention.

Referring again to FIG. 4, the present embodiment utilizes one or morepulse shapers 62 to shape the analog pulses received from each detector16 as required. Shaping is often necessary to account for ballisticdeficit and charge trapping, two effects associated with scintillationdetectors well known in the art. While the specific origins of each ofthese phenomena are documented in a number of publications, theireffects are of more significance to the present invention since theytend to distort the shape, timing, and amplitude of the pulses 61produced by the photomultiplier circuitry in the detectors 16.Specifically, ballistic deficit tends to broaden and delay the pulse,whereas charge trapping tends to distort the amplitude of the pulse,thereby reducing spectral resolution. Many commercially availabledetectors incorporate pulse shaping circuitry to allow compensation forthese effects. The pulse shaper(s) 62 of the present invention may be ofany type (such as, for example, those employing pulse integration, orthe “two pulse” method as disclosed in U.S. Pat. No. 5,021,664) whichsufficiently mitigates the effects of ballistic deficit, chargetrapping, or other similar phenomena. The signal output 63 from thepulse shaper(s) 62 merely must be such that a sufficiently high degreeof spectral resolution can be obtained for purpose of discriminatinggamma lines attributable to individual elements (described furtherbelow).

In order to take advantage of the great computational capabilityinherent in modem digital processors and integrated circuits, the analogpulses are converted to binary digital data 65 representative of gammaray energy, as shown in FIG. 4. This conversion is accomplished in thepresent embodiment through a standard analog-to-digital converter (ADC)64. For example, a standard 12-bit ADC, such as the TLC2500 seriesdevices manufactured by Texas Instruments Corporation, will provide morethan 4,000 possible “bins” (2¹² or 4096) for gamma energy resolutionwhile also allowing for multiple analog inputs. Assuming a gamma energyrange of 0-15 MeV, this allows for a spectral resolution ofapproximately 3.66 KeV. This level of energy resolution is more thanadequate for the purposes of the present invention (considering thespectral resolution capability of the Germanium detectors), and the12-bit ADC is compatible with a broad variety of commonly available FIFObuffers, logic gates, and other digital hardware as described furtherbelow.

After conversion to a multi-bit digital representation by the ADC 64,each pulse is passed through a digital pulse-height discriminator (PHD)66. Pulse height discrimination is used to eliminate or screen ranges ofthe detected gamma spectrum which are of little or no analytical value.For example, the PHD 66 can screen all pulses below a given desiredthreshold energy 67. In this fashion, the processing burden on eachdetector channel, and computational burden on the logic gate 72 can bereduced. Alternatively, the PHD 66 can be selectively configured to passall signals to the logic array such that a more complete spectrum can beanalyzed. The PHD 66 can be embodied in any of a wide variety ofhardware devices well know in the electronic arts, or may beconveniently implemented via the logic gate 72, as represented by thedashed lines between the PHD 66 and the logic gate 72 in FIG. 4.

Conceptually similar to pulse height discrimination described above,filtering in the context of the present invention relates to thefiltering out of specific pulses having unwanted gamma ray energies,such as those associated with inelastic scattering of neutrons fromelements invariant as a function of stressor application, thoseassociated with intervening materials (such as aluminum or steelconduit), or those not associated with inelastic neutron scattering(such as background radiation). The binary digital format of each pulseafter conversion by the ADC 64 is well suited to rapid discriminationand filtering by the logic gate 72, since the logic gate may be easilyprogrammed to efficiently eliminate data stored in memory arrayaddresses associated with unwanted discrete gamma energies. For example,if the gamma energies associated with the 400^(th) and 4000^(th) bins ofthe spectrum must be filtered, the logic gate can simply “skip over”these addresses when reading data out to the second stage buffer, asdescribed further below. It should be noted that pulse heightdiscrimination (as well as filtering) can be accomplished after thefirst stage buffer 68, using a similar approach. One possible detrimentto this approach, however, is that the first stage buffers must thenprocess and store data associated with all gamma energies as opposed toa greatly reduced set when pulse height discrimination is performedprior to the first stage.

The present embodiment of the analyzer 60 utilizes a form of memoryindirect addressing as further described herein. Referring now to FIG.5, the logic gate 72 stores data associated with the incoming pulsestream within its internal memory array 90 (“X” array) based on thebinary value of the data produced by the ADC 64. Each memory location isthen “indexed” (i_(n)) or incremented upon receipt of additional datawith the same address. In this way, the data stream is scaled using aminimum amount of memory space.

A secondary array 92 (“Y” array) located within the same or differentphysical memory is created based on the gamma energy values desired tobe filtered. This secondary array is generated based on previousobservations and analysis of energy spectra obtained from similar oridentical test specimens. For example, if it is known that inelasticscattering associated with Aluminum (a non-degradable material) in thecable conduit produces spectral lines at a set of different gammaenergies, these energies can be programmed into the Y array and filteredas the X array is read out of memory. The counting or scaling interval(i.e., number of “counts” obtained in order to produce a given spectrum)can be set to any value consistent with the memory resources of thelogic array 72 (or external memory, if used).

It is further contemplated that the present invention may be configuredto identify specific degradable materials or constituents thereofpresent in the test specimen through comparison of test data to apredetermined “signature” gamma spectrum associated with a givenmaterial and stored within the logic array 72 or other memory. In oneembodiment, the logic array 72 is programmed to obtain signature gammaspectrum data from the host PC 80 or external memory array anddifference the index value for each gamma energy bin. This produces atype of difference spectrum which can then be analyzed (either manuallyor via an algorithm within the host PC 80) to determine the level orquality of match between the observed spectrum and the signaturespectrum.

A field programmable gate array (FPGA) or application-specificintegrated circuit (ASIC) with embedded memory is chosen as the logicgate 72 since it may efficiently perform the relatively simple tasksnecessary to index and filter the digital data as described above, andno significant external is needed in the present application.Alternatively, if more sophisticated processing of the data is required(such as Fourier transform or other operation requiring a MAC stage), amore capable integrated circuit (such as a DSP having an external memoryinterface and DMA) may be utilized.

Referring again to FIG. 4, a first stage FIFO (first-in, first-out)buffer 68 and associated buffer manager 70 are used in each detectorchannel 82 to allow asynchronous storage and retrieval of spectral dataobtained from each detector. This architecture is utilized primarily toprevent data loss during data collection when using a comparatively highneutron flux, which produces a high gamma detection event rate. Crystaldetectors generally saturate at count rates on the order of 1E05-1E06cps, and may begin to suffer severe degradation of spectral resolutionat lower count rates. Based on a neutron flux of 1E06 n/s-4π, the gammacount rate for the present invention (each detector, includingbackground) is calculated to be well below saturation and spectraldegradation levels. However, backend signal processing as describedherein may, under certain circumstances, act as a “bottleneck” to dataoutput from the ADC 64 at very high ADC sampling rates. The sample rate(SR) of the ADC(s) 64 is set higher than the maximum anticipated eventor data rate (DR) rate to prevent data loss. Use of a first stage bufferallows for the accumulation of data between the ADC output and logicgate 72, thereby permitting use of a lower MIPS processor or logic gate72 or alternatively, use of a high MIPS device and much additionalprocessing of each data pulse. Use of second stage buffer 76 allows forthe accumulation of data between the logic gate output and thestorage/display device (personal computer) 80.

Data output from the PHD 66 is input to the first stage FIFO buffer 68under control of the buffer management module (BMM) 70. Each buffer 68may be further equipped with a separate overflow buffer 69, the buffermanager 70 monitoring the level within each primary buffer 68 andallocating data as necessary to the overflow buffer(s) 69 to preventdata loss. Such arrangement may be embodied in separate physicaldevices, or incorporated within a single piece of silicon. A TexasInstruments SN74ACT series device is chosen for the FIFO buffer of thepresent embodiment, although a wide variety of devices may be used withequal success. The logic gate 72 provides the necessary control signalsto the buffer manager via it's control port to read out data from thebuffer(s) 68 for further processing by the gate 72.

Data output from the logic gate 72 is input to the second stage buffer76 under control of the second stage buffer management module (BMM) 78.Again, the buffer(s) 76 may be provided with a separate overflow buffer77, the buffer manager 78 monitoring the level within the primary buffer76 and allocating data as necessary to the overflow buffer(s) in similarfashion to the first stage. The PC 80 provides the necessary controlsignals to the second stage buffer manager 78 to read out data from thebuffer(s) 76 for further processing, storage, or display by the PC 80. Astandard parallel data interface 89 or I/O adapter board of the typewell known in the computer art is used to interface the analyzer 60 withthe PC 80.

Degradation of Materials

For purposes of the present disclosure, the term “degradable” shall meanany material or object whose chemical or physical composition changes,in whole or in part, as a result of the application of one or morestressors. Stressors as used herein refers to any chemical, electrical,physical or other force or influence including, without limitation, heat(whether by conduction, convection, or radiation), nuclear and cosmicradiation, electrical potential or current, chemicals, oxygen and othergases, volatization, or any combination thereof.

The primary constituent atoms within most commercially availablepolymers include carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine,and fluorine. For example, in electrical cable insulation and jacketing,materials such as Hypalon™ (CSPE, or chlorosulfonated polyethylene), EPR(ethylene propylene rubber), PVC (polyvinyl chloride), Tefzel™ (ethylenetetraflouroethylene) and XLPE (crosslinked polyethylene) are quitecommon. In addition to the base polymers listed above, many materialscontain a variety of other substances or compounds which perform variousancillary functions. For example, lamp black (carbon) is commonly addedto polyethylene in order to increase its resistance to cracking anddegradation due to ultraviolet radiation. Clay (or other similarmaterial) is commonly used as filler, often comprising the majoritycomponent within electrical cable insulation/jacketing in order toreduce cost. Plasticizers are commonly added to polymer formulations(including most notably PVC and CSPE) to increase their pliability andresistance to fatigue cracking. A typical formulation of EPR mightconsist of EPDM (ethylene propylene diene monomer), parrafin wax, zincsalts and oxides, vinylsilane, diadduct of hexachlorocyclopentadiene,dicumyl peroxide, SRF black, and antimony oxide.

Many polymer formulations also contain additives specifically designedto reduce the flammability of the cable insulation/jacketing undercertain conditions. These so-called “fire retardants” volatize tovarying degrees under exposure to heat and radiation, and are emittedfrom the cable at a rate related at least in part to thetemperature/radiation dose rate to which the material is exposed. Inmany materials, the volatization of fire retardants roughly parallelsthe volatization of other flammable compounds; hence, the overallflammability of the material remains roughly constant as a function ofthermal and/or radiation aging. However, as the fire retardants andflammable compounds are removed from the material, the relativeconcentration of the constituent atoms of these substances within thematerial as a whole change.

Similar to fire-retardants discussed above, plasticizers used in variouspolymer formulations are lost from the material as a function of agingand stressor application. Plasticizers are lost via both volatizationand scission of the molecule. Plasticizer content has been shown to havea good correlation with, inter alia, elongation-at-break of certainmaterials in the early stages of component aging. Later in life,however, plasticizer content remains essentially constant for manymaterials, thereby limiting the effectiveness of these compounds asaging indicators during this period.

Ozone (O₃) is another stressor which may act on certain materials. Ozoneis generated in the air as a result of the interaction of ionizingradiation with monatomic or diatomic oxygen, or by corona dischargeionization. Similar to oxygen diffusion, ozone effects occurpredominately at the surface of the object where the ozone concentrationis highest. Generally, however, most modern polymer formulations areresistant to the effects of ozone.

Cable components may also be exposed to chemical by-products of thethermal or radiolytic decomposition of cable jacketing, insulation,fire-resistant coatings, or other organic components. Many materialscommonly used in cable construction either contain or are manufacturedusing potentially corrosive chemicals such as chlorides, peroxides, orsulfurous compounds. Chemical by-products originating from decompositionof cable components may result in several degradation mechanisms,including softening, swelling, or decomposition of other organics withinthe cable structure. Plasticizer migration (PVC) can also result inswelling of adjacent elastomers.

For example, neoprene rubber (chloroprene), PVC (polyvinyl chloride),CSPE (chlorosulfonated polyethylene), and CPE (chloropolyethylene) mayall produce chlorine ions (and hydrochloric acid) upon decomposition.Additionally, elastomers including EPR/EPDM are cured using peroxide orsulfur compounds that can be leached from the material as it ages or issubjected to certain environmental conditions (such as heat or wetting).Copolymers such as ethylene vinyl acetate (semiconducting shieldmaterial) may also decompose to produce by-products such as weak acids.

Degradation resulting from copper-catalyzed oxidation reactions mayoccur in certain polymers as well. A catalyst is defined as a substancethat affects the rate or the direction of a chemical reaction, but isnot appreciably consumed in the process. Because of its proximity to theinsulation, ions from copper-based conductors may act as catalysts foroxidation reactions within the insulation, thereby accelerating itsdegradation. This will occur primarily in areas where the insulation isin direct contact with the conductor.

By-products are also generated from chemically crosslinked XLPE as aresult of the high temperature/pressure curing process. By-products suchas acetophenone, cumene, and alpha methyl styrene are produced as thechemical crosslinking agent (dicumyl peroxide) decomposes.

Another potential aging mechanism is hydrolytic degradation of mylar(polyethylene terephthalate) shield film under exposure to hightemperature and moisture. Under this mechanism, water increasinglyreacts with the mylar polymer as temperature is increased.

In sum, there are a substantial number of different possible agingmechanisms for electrical cable components (and more broadly, degradablecomponents), each of which may ultimately vary the concentration ofvarious atomic species within the material. The effects of these agingmechanisms are specific to each class or even formulation of material,and hence generally must be analyzed individually. For example, asdiscussed above, it can be shown for some materials that the rate offire retardant loss is roughly proportional to the thermal aging appliedto the material (at least for certain aging intervals). Hence, thesignature gamma lines associated with the specific fire retardantpresent in that material are used as an indirect indicator of thermalaging. Since fire retardant volatizes, the atomic concentration andhence inelastic scattering of neutrons by the constituent atoms (whichmay include carbon, hydrogen, bromine, fluorine, or chlorine) will alsovary as a function of aging. As the atomic concentration (N) of a givenelement is reduced, the associated gamma yield at specific energies isreduced as well (assuming a measurable gamma yield for the incidentneutron energy selected). Hence, a reduction in atomic concentration dueto aging stressors is ultimately reflected as a reduction in detectedcounts at those energies as compared to prior spectra of the samesample; see FIGS. 6a and 6 b, which show typical gamma spectra takenfrom the same specimen at two different levels of aging. The followinggeneralized formula represents the approximate gamma counting rate for agiven material, gamma energy, and neutron energy in the apparatus of thepresent invention (assuming no detector or processing circuitsaturation):

CR=S·dφ·E_(d)·γ_(i)·AF

Where:

CR=Counting Rate (cps)

S=Total neutron source strength (neutrons/s-4π)

dφ=Uncollimated solid angle subtended by active detector area(steradians)

E_(d)=Detector efficiency at selected gamma energy

γi=Gamma yield for ith material for selected gamma energy and incidentneutron energy

AF=Attenuation factor for interposed materials for selected gamma energy

Note that the gamma yield γ_(I) as shown in the above relationship is acomplex function of the inelastic scattering cross-section (σ) of thevarious constituent atoms, their atomic concentrations (N), and theincident neutron energy. Obviously, the yield at different gammaenergies will vary for each material.

The present invention further contemplates the evaluation of multipledegradation processes during the installed lifetime of the degradablecomponent as required. For example, while changes in the gamma spectrumassociated with plasticizer loss may be useful during the earlier stagesof component life, fire retardant volatization may be a better indicatorof component condition later in life.

Neutron and Gamma Attenuation in Surrounding Materials

One of the principal benefits of the present invention is its ability to“look through” most any components or materials interposed between thetest subject 11 and neutron source 12. This unique property results fromthe use of energetic neutrons which have a very lowscattering/absorption cross-section in most materials of relatively lowthickness (i.e., less than a few inches). Obviously, some attenuation ofthe incident neutron beam will occur. Unlike the interaction of gammarays with matter (described below), the energy and spatial distributionof incident neutrons will vary as a function of the attenuatingmaterial. Specifically, a fraction of the neutrons in the incident beam13 will be reduced in energy, and a fraction will be scattered at anglesrelative to the beam centerline. This characteristic is due not tocoulombic interaction, but rather the inelastic scattering of thecomparatively massive neutron with other particles in a given nucleus.The neutron spatial and energy distributions after passage through anintervening material are not critical in the present embodiment of theinvention, since 1) a sufficient population of sufficiently energeticneutrons will exist after passing through most any material in mostcontemplated applications; 2) gamma rays (and not neutrons) are detectedupon their egress from the test subject; and 3) the spatial position ofthe test subject is not being measured, hence, any errors induced byalteration of the spatial distribution of neutrons will only affect thescope of material within the test subject 11 which is analyzed. Thisaffords the invention the ability to analyze test subjects shieldedbehind any number of types and configurations of materials. Theaforementioned secondary gamma emissions are prompt (occur on the orderof femtoseconds after scattering) and spatially distributed around thetarget atom(s).

Unlike neutrons, gamma rays (photons) generally retain their initialenergy regardless of intervening material; rather, such materials act toattenuate the gamma flux, yet not alter the spectral or spatialdistribution. Lower energy gammas are attenuated much more severely byrelatively thin materials than are higher energy gammas. For example,the attenuation of 100 KeV gamma flux in 1 inch of steel is almostcomplete, whereas the attenuation of a 10 MeV gamma flux in the samematerial is fairly minimal. A common measure of this property isso-called “tenth thickness”, defined as the thickness of a givenmaterial required to attenuate an incident gamma flux of a given energyto one-tenth of it's initial value.

Damage to the test subject 11 and any intervening material resultingfrom incident neutron radiation during testing with the apparatus 10described herein is not considered significant, since the totalintegrated dose (TID) applied to a given test specimen even over severalin-situ tests is well below the threshold dose necessary to result inmeasurable property changes in the target. Most elastomers,thermoplastics, and thermosets have estimated neutron threshold doses onthe order of 1E14 n/cm2, whereas the neutron dose imparted during astandard testing protocol of the present invention is several orders ofmagnitude below this value. Even when accounting for differences inneutron energy, the dose to a given component resulting from evenfrequent periodic testing during its lifetime is well below thethreshold value cited above. Furthermore, with certain test components(such as cable), slightly different physical locations on the cable withessentially identical environmental conditions may be used forsubsequent tests in order to distribute the neutron/gamma dose withinthe component. A tightly collimated neutron beam (given the same fluxemitted from the source 12 prior to collimation by the collimator 14)allows greater spatial control, yet generally at the expense of slowercounting rates and longer integration times.

Aging Analysis and Method

As is presently known in the art, specimens of similar or identicalconstruction to the in-situ components to be analyzed may be aged in acontrolled fashion in order to observe changes in various physical,chemical, electrical, or atomic characteristics. Typically, the aging isaccelerated in nature (in order to make the results available moreimmediately), and attempts to replicate the environmental conditions towhich the component will be exposed as closely as possible. Theselaboratory or artificially aged specimens are then used as “yardsticks”against which in-situ components are compared in order to evaluate thecondition (level of aging) of the component.

The present invention utilizes the foregoing general approach toevaluate in-situ specimens as depicted in FIG. 7. First, a specimensimilar or identical to the in situ component being evaluated is aged,either naturally or artificially, in a manner consistent with thenatural aging of the in-situ component. For example, if the in-situcomponent is exposed to heat and radiation, similar aging is applied tothe specimen to induce similar types of degradation. The specimen isaged to or beyond the maximum expected level of aging anticipated forthe in-situ component to provide a complete aging profile. Thetechniques used to artificially and naturally age specimens for purposesof in-situ component aging analysis are well known and understood in theart, and accordingly need not be explained further herein. At selectedintervals during the specimen aging process, gamma spectra are obtainedfrom the specimen using the above-described apparatus and stored withinthe memory of the analyzer 60 or host PC 80 for later use. For example,the spectra (or sets of spectra) may be obtained at 25, 50, 75, and 100%of anticipated aging of the specimen. The spectra are also analyzed toidentify gamma energies associated with non-degradable components withinthe specimen, such as the copper conductors of the cable. These gammaenergies are recorded for later use with the pulse heightdiscrimination/filterfunctions of the analyzer 60 previously described.It should be noted that essentially the entire gamma spectrum obtainedfrom the specimen is used in the analysis, since as previouslydescribed, different spectral lines may be more or less useful as afunction of the level of aging.

Next, the apparatus of the present invention is again used to obtain afirst spectrum (or set of spectra if averaging or more complexstatistical analysis is used) of the in-situ component at a given timein the lifetime of that component. In the case of intervening materialssuch as conduit or valve bodies, the representative spectra obtainedfrom the insitu component is differenced, using the analyzer 60described above, from that of the laboratory aged specimen for acomparable level of aging in order to identify the spectral linesassociated with the intervening material. The choice of specimenspectrum for comparison to that obtained from the in-situ component isnot critical (so long as a spectrum representing a roughly comparablelevel of aging is chosen), since variations in the aging of thedegradable materials will be minimal in comparison to the more salientdifferences relating to the non-degradable materials. The gamma energiesassociated with these “salient” differences are then entered into thefiltering algorithm (i.e., the “Y” array values) to permit filtering ofsubsequent spectra obtained with the intervening material in place. Notethat multiple arrays (filter values) may be stored in memory andaccessed as desired depending on the specific application.

Lastly, a second spectrum (or set of spectra) is obtained from thein-situ specimen at a later time in life, or after the application of asignificant stressor. This second spectrum is then filtered as necessaryand compared to the First spectrum in order to identify changes in thematerial as a function of aging/stressor application. Specifically, thedifferences between the first and second spectra from the in-situspecimen are compared to the difference between the gamma spectrum forthe unaged laboratory specimen and those taken at subsequent timesduring the artificial (or natural) aging regimen as shown in FIG. 8. Forexample, if the difference spectrum for the in-situ specimen indicates achange of 1000 cps for a gamma energy of 3 MeV, and the 25% and 50%aging spectra for the laboratory specimen indicate changes at 3 MeV of500 cps and 2000 cps, respectively, the aging of the in-situ componentcan be inferred to be between 25% and 50%. Obviously, more sophisticateddifferencing and analytical interpolation techniques may be employed tomore precisely estimate the level of aging of the component; theforegoing example is merely illustrative of the general methodology ofthe present embodiment. Note also that such comparisons may befunctionally implemented entirely in software operating on the PCprocessor 80, such software being easily produced using techniques wellknown in the computer arts.

It should be recognized that while the foregoing discussion hasdescribed a specific sequence of steps necessary to perform the methodof the present invention, other sequences (such as obtaining the in-situmeasurements prior to conducting artificial aging of the laboratoryspecimens) of steps may be used depending on the particular application.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. The foregoing description is of the best mode presentlycontemplated of carrying out the invention. This description is in noway meant to be limiting, but rather should be taken as illustrative ofthe general principles of the invention. The scope of the inventionshould be determined with reference to the claims.

What is claimed is:
 1. An apparatus for estimating the degradation of adegradable object over time, comprising: at least one neutron sourceadapted to irradiate at least a portion of said object; at least oneradiation detector adapted to detect radiation emitted from said atleast portion of said object resulting from said irradiation by said atleast one neutron source; and an analyzer operably connected to said atleast one radiation detector, said analyzer being adapted to analyzesaid radiation emitted from said at least portion of said object at twoor more different times in order to estimate the degradation of saidobject.
 2. The apparatus of claim 1, wherein said apparatus isstructurally adapted to estimate the degradation of a cable.
 3. Theapparatus of claim 1, wherein said analyzer compares at least onespectral line present within each of at least two gamma spectra in orderto evaluate the degradation of said object, said at least one spectralline taken from the group consisting of spectral lines associated withatomic species of oxygen, carbon, and nitrogen.
 4. The apparatus ofclaim 3, wherein said analyzer further comprises a multi-channel gammaspectral analyzer.
 5. The apparatus of claim 4, wherein said radiationdetector is a high-resolution gamma detector.
 6. The apparatus of claim1, wherein said neutrons comprise fast neutrons.
 7. The apparatus ofclaim 1, wherein said apparatus is adapted to difference gamma spectraobtained from the same degradable object at said two or more differenttimes, and estimate the degradation of said object based at least inpart thereon.
 8. The apparatus of claim 7, wherein said apparatus isconfigured to estimate the degradation of a degradable object that isphysically contained substantially within another object.
 9. Theapparatus of claim 1, further comprising a collimator capable ofattenuating at least a portion of the neutrons emitted by said at leastone neutron source.
 10. The apparatus of claim 9, further comprising: atleast one neutron shield capable of attenuating at least a portion ofthe neutrons generated by said at least one source; and at least onegamma ray shield capable of attenuating at least a portion of theradiation emitted by said at least portion of said object afterirradiation by said at least one neutron source.
 11. The apparatus ofclaim 10, wherein at least a portion of said at least one gamma shieldis disposed between said at least one neutron shield and said degradableobject.
 12. The apparatus of claim 9, wherein said apparatus is furtheradapted to estimate the degradation of said degradable object as afunction of spatial location within at least a portion of said object.13. The apparatus of claim 1, further comprising at least one moveablearm coupled to said at least one radiation detector and adapted topermit positioning of said at least one detector in a plurality ofpositions with respect to said degradable object.
 14. The apparatus ofclaim 1, wherein said at least one neutron source is adapted to generateneutrons at two or more neutron energy levels, said neutrons generatedat said two or more energy levels being used to irradiate said at leastportion of said object.
 15. An apparatus for estimating the condition ofa degradable object over time, comprising: means for irradiating atleast a portion of said object with a neutron flux; radiation detectionmeans for detecting gamma radiation emitted from said at least portionof said object resulting from said irradiation by said means forirradiating; and means for analyzing said gamma radiation emitted fromsaid at least portion of said object at two or more different times inorder to estimate the condition of said object, said means for analyzingbeing operably connected to said at least one radiation detection means.16. A method of estimating the aging of a degradable object, comprising:irradiating at least a portion of said object with neutrons at a firsttime, a second time and a third time; detecting first, second and thirdradiation emitted from said object resulting from said act ofirradiating at said first, second and third times, respectively;generating first, second, and third radiation spectra based on saidfirst, second and third radiation emissions, respectively, said first,second, and third spectra having a first, second, and third plurality ofspectral lines, respectively, each of said spectral lines correspondingto at least one atomic species, respectively; comparing said second andfirst spectra to estimate the aging of said degradable object occurringbetween said first and second times; and comparing said third and secondspectra to estimate the aging of said degradable object occurringbetween said second and third times.
 17. The method of claim 16, whereinsaid act of comparing said third and second spectra comprises comparingthe changes in at least one spectral line which is not compared as partof the act of comparing said second and first spectra.